Add signature to mjSpec and mjModel and use it to perform safe bind to mjModel and mjData.

PiperOrigin-RevId: 740378879
Change-Id: If14b326942529494f172e7aedcae30195798b458

include/mujoco/mjmodel.h

@@ -1155,6 +1155,9 @@ struct mjModel_ {
 
   // paths
   char*     paths;                // paths to assets, 0-terminated            (npaths x 1)
+
+  // compilation signature
+  uint64_t  signature;            // also held by the mjSpec that compiled this model
 };
 typedef struct mjModel_ mjModel;